The present invention relates to a membrane-electrode-assembly with a solid polymer electrolyte and to a manufacturing method thereof, and more particularly to an improved catalyst layer structure for use in a membrane-electrode-assembly with a solid polymer electrolyte.
FIG. 1(a) depicts the basic structure of a fuel cell using a membrane-electrode-assembly with a solid polymer electrolyte. A solid polymer electrolyte 1 is sandwiched between an anode 2 and a cathode 3, and gas diffusion layers 4 and 5 are formed on the outside of the anode 2 and cathode 3. On the anode side, hydrogen ions (protons) and electrons are produced by the catalyst constituting the anode 2 from a hydrogen gas fed to the anode 2 through the gas diffusion layer 4, and the resulting hydrogen ions pass through the solid polymer electrolyte 1 and form water by reacting with an oxygen gas fed to the cathode 3 via the gas diffusion layer 5 on the side of the cathode 3 and with electrons fed to the cathode 3 through outside circuitry.
Anode: H2xe2x86x922H++2exe2x88x92
Cathode: xc2xdO2+2H++exe2x88x92xe2x86x92H2O
The solid polymer electrolyte 1 may, for example, be a solid polymer electrolyte membrane composed of a membrane based on perfluorosulfonic acid such as an ion-conducting resin typified by a Nafion(copyright) polymer. The ability to form this polymer into a membrane is well known in the art, described, for example, in xe2x80x9cProcedure for Preparing Solution Cast Perfluorosulfonate lonomer Films and Membranes,xe2x80x9d R. B. Moore and C. R. Martin, Anal. Chem., 58, 2569 (1986), and in xe2x80x9cIon Exchange Selectivity of NAFION(copyright) Films on Electrode Surfaces,xe2x80x9d M. N. Szentirmay and C. R. Martin, Anal. Chem., 56, 1898 (1984).
It is also known to form stronger and thinner ion conducting membranes by reinforcing the ion-conducting polymer. In U.S. Pat. No. 5,547,551 and U.S. Pat. No. 5,599,614 to Bahar et al a composite structure of an ion conducting material contained in a base material characterized by the presence of nodes interconnected by fibrils is described. This membrane can be prepared much thinner than the ion-conducting polymer alone while still retaining enough strength for handling and use. These thinner membranes can offer improved cell performance because there is reduced cell resistance, and therefore less power loss during fuel cell operation.
The anode 2 and cathode 3 should preferably be composed of a catalyst capable of promoting the necessary electrode reactions. The composition of the catalyst used in the anode and cathode are well known in the art. Typically, some form of dispersed Pt is used in the anode, often in the form of Pt on carbon particles, while the catalyst in the cathode is typically also a Pt or Pt alloy, again often dispersed on finely grained carbon particles. Often, the catalyst is combined with an ion-conducting material or other binders and subsequently applied to the SPE membrane. Additionally, it is known in the art that one can also provide a catalyst-containing layer on the gas diffusion media.
The use of bi-layer electrodes have been described by Wilkinson in U.S. Pat. No. 5,795,669. Wilkinson""s teachings are directed toward improved poisoning resistance. He disclosed the use of a two layer electrode, where one is specifically tailored to be electrochemically active, i.e., includes the presence of an ionomer, and one of which is specifically tailored to be active only in the gas phase, i.e., does not contain an ionomer. Wilkinson specifically teaches the advantage of this electrode arrangement for reducing the concentration of poisoning species in the gas phase. The layers are formed sequentially one on top of the other. The catalyst in each of the two layers is also different. The presence of the gas-active catalyst is taught as being capable or reducing the effect poisons present in the gas phase on the electrochemical reaction catalyst
The gas diffusion layers 4 and 5 are composed of a material having electric conductivity and gas permeability, such as carbon paper, woven fabric, nonwoven fabric, or another material consisting of carbon fibers.
A membrane-electrode-assembly with a solid polymer electrolyte can be easily manufactured by a method in which a solution containing catalyst particles and an ion-conducting resin is applied to the surface of a gas diffusion layer obtained using carbon paper, woven fabric, nonwoven fabric, or another material consisting of carbon fibers; and the coated catalyst diffusion layer is dried, yielding a catalyst layer. A product (gas diffusion layer/catalyst layer conjugate) in which catalyst layers 2 and 3 containing catalyst particles and ion-conducting resins are formed on the surfaces of the gas diffusion layers 4 and 5 is commonly bonded by hot pressing or another technique on both sides of the solid polymer electrolyte membrane 1, as shown in FIG. 2. In preferred practice, a layer 6(7) composed of carbon-based particles and a fluorine-based resin (or ion-conducting resin) is disposed between the gas diffusion layer 4(5) and catalyst layer 2(3), as shown in FIG. 1(b) (fragmentary expanded view of section B in FIG. 1(a)). The same manufacturing method is used in this case. Referring again to FIG. 2, hot pressing or another technique is employed in this particular case to bond the gas diffusion layer/catalyst layer conjugate 8, 9 to the solid polymer electrolyte membrane 1 because the solid polymer electrolyte membrane 1 and the catalyst layers 2 and 3 need to be joined together with minimal contact resistance. It has therefore been proposed to use methods in which ion-conducting resin solutions are used as adhesives, methods in which the components are joined using solvents capable of dissolving solid polymer electrolyte membrane materials, and other methods in addition to the hot pressing, roll pressing, and other thermocompression methods typically employed as prior art (JP (Kokai) 7-220741, 8-148167, etc.).
The catalyst layers 2 and 3 are sometimes formed directly on the surfaces of the solid polymer electrolyte membrane 1 by spraying, screen printing, decal transfer (in which the catalyst layers are thermally transferred after being formed on PTFE sheets or the like), and other methods, as shown in FIG. 3. In such cases a membrane-electrode-assembly with a solid polymer electrolyte is constructed by combining a membrane/catalyst layer conjugate 10 with the gas diffusion layers 4 and 5.
When, however, the gas diffusion layer/catalyst layer conjugate is bonded under heat and pressure to a solid polymer electrolyte membrane after being formed, physical or chemical damage may sometimes occur as a result of heating in the membrane itself or in the gas diffusion layers during hot pressing or another type of thermocompression bonding due to the recent trend for using thinner solid polymer electrolyte membranes. Another drawback is that because this method joins the solid polymer electrolyte membrane and the catalyst layers only slightly and yields a two-dimensional contact, the high contact resistance and physical or chemical damage to the membrane itself result in the poor performance and reduced durability of the membrane-electrode-assembly. In addition, methods in which the gas diffusion layer/catalyst layer conjugate thus formed is bonded to the solid polymer electrolyte membrane with the aid of a solution or solvent are disadvantageous in that the solid polymer electrolyte membrane is dissolved in the solution or solvent and is thus more likely to be damaged, yielding a membrane-electrode-assembly whose performance is compromised in the same manner as above.
On the other hand, methods in which catalyst layers are directly formed on the surfaces of a solid polymer electrolyte membrane are expected to provide good bonding between the solid polymer electrolyte membrane and the catalyst layers and to allow membrane-electrode-assemblies to perform better than when a gas diffusion layer/catalyst layer conjugate is used.
However, even these methods fail to provide adequate joining between catalyst layers and gas diffusion layers or to bring about sufficiently low contact resistance when a membrane/catalyst layer conjugate is fabricated. A resulting drawback is that resistance increases due to the accumulation of water (flooding) along the interface with the catalyst layer on the cathode side, accompanied by impeded mass transfer of reaction gas, product water, and the like. Methods for reducing such contact resistance by raising the contact temperature or pressure during hot pressing or the like or by increasing the pressure with which membrane-electrode-assemblies are tightened during assembly have therefore been proposed in order to establish better contact between catalyst layers and gas diffusion layers, but these methods, while capable of reducing contact resistance, still fail to deliver satisfactory long-term cell performance because of the possibility that thin solid polymer electrolyte membranes can be physically or chemically damaged by heat or pressure.
Despite the art described above, there remains a distinct need within the industry for more efficient and more durable fuel cell components. In order for solid polymer electrolyte fuel cells to become widely used they must be able to operate with a high power output, and to maintain that operation with little or no degradation in performance. It is therefore an object of the present invention to provide an improved membrane electrode assembly that, when used in a fuel cell, offers both improved initial performance, and an improved ability to maintain that performance advantage. An additional object of the present invention, which was perfected in view of the drawbacks of the prior art described above, is to provide the desired high-performance, durable membrane-electrode-assembly with a solid polymer electrolyte using a manufacturing method that will reproducibly and consistently provide the desired membrane electrode assemblies.
The present invention allows the stated object to be attained in the following manner.
The instant invention is a solid polymer electrolyte membrane-electrode-assembly, comprising an anode-side gas diffusion layer, an anode catalyst layer, a solid polymer electrolyte membrane, a cathode catalyst layer, and a cathode-side gas diffusion layer in a sequential arrangement. (In the context of this invention, the membrane-electrode-assembly includes gas diffusion media as well as the membrane and the electrodes.) This membrane-electrode-assembly with a solid polymer electrolyte is characterized in that the anode or cathode catalysts region or both has at least two layers. The two layers or more layers are prepared so that at least one layer has one side attached directly or indirectly to the gas diffusion layer, and a second layer has at least one side attached directly or indirectly to the solid polymer electrolyte. Both of these layers are comprised of at least one catalyst component and at least one ionomer. This latter feature particularly distinguishes the instant invention from prior art because although prior art has described bi-layer electrodes, the use of an ionomer and a catalyst component in both layers was not taught or anticipated. Additionally, as will be described more fully below, this difference gives rise to totally unexpected performance advantages not described or anticipated previously.
When used in a fuel cell, this arrangement has, surprisingly, been found to yield fuel cells that give higher initial performance than prior art. Further, and even more surprisingly, the fuel cells using the instant invention are more stable during operation. Here stable means that the voltage decay is lower in the instant invention than in prior art. The voltage decay is the average voltage loss per unit time when the cell is operating under constant current conditions. It can be calculated using several different methods. Most simply, it is the change in voltage after some test time, t, from the initial open circuit voltage divided by the test time, t. Alternatively, it can be calculated by measuring a polarization curve at the start of a test, and then again at the end of the test. Electrochemical cell diagnostics well known in the art, for example cyclic voltammetry, may be performed if desired before obtaining each of these polarization curves. The voltage at a given current density is extracted from the two polarization curves, and the decay rate is calculated according to:
xe2x80x83Decay Rate=(Voltage Beginningxe2x88x92Voltage End)/Hours of Test,
where xe2x80x9cVoltage Beginningxe2x80x9d is the voltage extracted at a given current density from the polarization curve taken at the start of the test, and xe2x80x9cVoltage Endxe2x80x9d is the voltage at that same current density used at the start of the test but extracted from the polarization curve taken at the end of the test. The particular method chosen to measure decay rate depends upon the nature of the testing protocol, which may depend upon the final application of the particular fuel cell being tested. To compare the stability between two different cells or two different cell configurations, it is only necessary to use the same method of calculating the decay rate between the two cells. As long as a consistent method of calculating the decay rate is used, the cell with the lower decay rate is considered to be more stable.
The lower decay rate of the instant invention is not only surprising, it is important and useful technically. One of the main limitations to prior membrane-electrode-assemblies and fuel cells that use them, is that they are not able to operate at high power outputs for long periods of time. With continued operation at a given output level as characterized by the current density, the voltage typically decreases continually, thereby eventually leading to a cell that produces little or no power. With the instant invention described herein, this limitation is greatly reduced, and therefore should allow a wider applicability for the use of membrane-electrode-assemblies in fuel cells and elsewhere.
The membrane-electrode-assembly with a solid polymer electrolyte can also be prepared such that the first catalyst layer of the membrane/catalyst layer conjugate and the second catalyst layer of the gas diffusion layer/catalyst layer conjugate are kept in contact with each other, with or without being joined. Herein, joined means brought in intimate contact using heat and/or high pressure for a period of time, for example by lamination. When the layers are not joined they are simply brought together using a relatively low mechanical force, for example by that provided during assembly of a fuel cell.
The membrane-electrode-assembly with a solid polymer electrolyte can also be prepared so that a catalyst layer of the gas diffusion layer/catalyst layer conjugate is bonded to the gas diffusion layer via a carbon-based particle layer composed of carbon-based particles and a fluororesin and disposed on one side of the gas diffusion layer. This indirect attachment of the layer to the gas diffusion media can be advantageous in certain circumstances as described more fully below. This carbon-fluororesin composite layer used for attachment can be prepared in numerous ways well known in the art, including by simple mixing and hand applications, or by using a membrane, for example expanded PTFE containing carbon.
The membrane-electrode-assembly catalysts contained in the first and/or second catalyst layer comprises particles that can be a variety of a noble metal catalyst particles, either as free standing catalyst with a high surface area, or preferably, supported on carbon particles.
The gas diffusion media (GDM) can be prepared from a gas-permeable electroconductive sheet material comprising a carbon fiber woven fabric, a carbon fiber nonwoven fabric, carbon felt, carbon paper, or any of these coated with a fluororesin containing carbon-based particles.
In another embodiment, this invention provides a method including:
a step for preparing a membrane/catalyst layer conjugate by bonding a first catalyst layer containing a catalyst and an ion-conducting resin to a solid polymer electrolyte membrane;
a step for preparing a gas diffusion layer/catalyst layer conjugate by forming a second catalyst layer containing a catalyst and an ion-conducting resin on one side of a gas diffusion layer composed of a gas-permeable electroconductive sheet material; and
a step for forming a laminated structure by laminating the membrane/catalyst layer conjugate and the gas diffusion layer/catalyst layer conjugate such that the first catalyst layer and the second catalyst layer face each other, with the laminated structure being used at least on the anode or cathode side.